1. Field of the Invention
This invention relates to a process for forming a zinc oxide film, an apparatus for forming a zinc oxide film, and a process, and an apparatus, for producing a photovoltaic device by using the same.
2. Related Background Art
Photovoltaic devices comprised of amorphous silicon hydride, amorphous silicon germanium hydride, amorphous silicon hydride carbide, microcrystalline silicon or polycrystalline silicon are conventionally provided with reflecting layers on their backs in order to improve light-collection efficiency in the long-wavelength regions. It is desirable for such-reflecting layers to show effective reflection characteristics at wavelengths which are close to band edges of semiconductor materials and at which absorption becomes small, i.e., wavelengths of 800 nm to 1,200 nm. Those which can fulfill such a condition are reflecting layers formed of a metal such as gold, silver, copper or aluminum. It is also prevalent to provide an uneven layer which is optically transparent within a stated wavelength region. This transparent uneven layer is provided between the metal layer and a semiconductor active layer so that reflected light can effectively be utilized to improve short-circuit current density Jsc. In order to prevent characteristics from lowering because of shunt pass, it is still also prevalent to provide between the metal layer and a semiconductor layer a layer formed of a light-transmitting material showing a conductivity, i.e., a transparent conductive layer. This transparent conductive layer and the above transparent uneven layer may be the same layer. In general, these layers are deposited by a process such as vacuum evaporation or sputtering and show an improvement in short-circuit current density Jsc by 1 mA/cm2 or above.
As an example thereof, in xe2x80x9cLight Entrapment Effect in a-Si Solar Cells on 29p-MF-22 Stainless Steel Substratesxe2x80x9d (autumn, 1990), The 51st Applied Physics Society Scientific Lecture Meeting, Lecture Drafts p.747, xe2x80x98P-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells with Bandgap Profilingxe2x80x99, Sannomiya et al., Technical Digest of The International PVSEC-5, Kyoto, Japan, p.381, 1990, reflectance and texture structure are studied on a reflecting layer constituted of silver atoms. In this example, it is reported that the reflecting layer is deposited in double layer of silver by changing substrate temperature, to form effective unevenness, which has achieved an increase in short-circuit current in virtue of light entrapment effect.
The transparent layer used as a light entrapment layer is deposited by vacuum evaporation utilizing resistance heating or electron beams, sputtering, ion implantation or CVD (chemical vapor deposition). However, the facts of high wages for preparing target materials and so forth, a large repayment for vacuum apparatus and not a high utilization efficiency of materials make very high the cost for photovoltaic devices produced by these techniques, and put a high barrier to industrial application of solar cells.
As a technique for forming a zinc oxide film by electrodeposition from an aqueous solution, intended to solve these problems, Japanese Patent Application Laid-Open No. 10-178193 discloses its combination with a metal layer and a transparent conductive layer which are formed by sputtering, applied as a reflecting layer of photovoltaic devices (solar cells). Also, as an improved technique of such a zinc oxide production technique, Japanese Patent Application Laid-Open No. 11-286799 by the present inventors discloses a technique in which a back-side film adhesion preventive electrode is provided to form zinc oxide films by electrodeposition causative of less or no back deposition.
These methods do not require any expensive vacuum apparatus and any expensive targets, and can dramatically reduce the production cost for zinc oxide films. These also enable deposition on a large-area substrate, and are full of promise for large-area photovoltaic devices such as solar cells. However, these methods of making deposition electrochemically have matters to be improved, as shown below.
1) The back-side film adhesion preventive electrode removes any unwanted electrodeposited film (zinc oxide film) deposited on the back surface, but in that course the zinc oxide film removed keeps depositing on the back-side film adhesion preventive electrode. After its use for a long time, the deposited film comes off from the back-side film adhesion preventive electrode because of film stress or the like and film pieces fall on the substrate. The zinc oxide film having deposited on the back-side film adhesion preventive electrode is of hard and brittle nature. Hence, when film pieces are held between a roller and a substrate to become crushed, they are broken into sand. This zinc oxide broken into sand adheres to the transport roller surface to cause the occurrence of impact marks continually.
2) Where such film pieces or adsorbed matter have or has passed the roller as they are, crushed film pieces not only cause the occurrence of impact marks continually but also cause the occurrence of dents, scratches and so forth at the roller surface to make the roller have a short lifetime, resulting in a great damage on the apparatus.
3) Dust of white powdery zinc oxide coming out from the anode also drifts or floats in the bath. Such dust is so treated as to be removed with a filter attached to the apparatus. However, before it is removed with the filter, it becomes adsorbed on the film-deposited substrate to cause impact marks when it passes the transport roller as it is.
4) The impact marks having thus occurred not only make poor the surface appearance required as solar cell substrates, but also may cause cracks and film-peeling in zinc oxide film deposited by electrodeposition. Where such zinc oxide film is used as a part of a photovoltaic device, the cracks and film-peeling cause shunt pass of the photovoltaic device, as so considered.
Accordingly, the present invention was made taking account of such circumstances, and an object of the present invention is to establish a technique for mass production of zinc oxide films by electrodeposition, and, in an electrodeposition process combined with substrate transportation of a roll-to-roll system or the like, to prevent occurrence of the above impact marks to stably form high-quality and low-cost zinc oxide films over a long period of time, the films being free of cracks and film-peeling, so as to contribute to real spread of sunlight electricity generation by incorporating such zinc oxide films in photovoltaic devices.
Another object of the present invention is, without limitation to the formation of zinc oxide films, to provide a process, and an apparatus, for forming good-quality electrodeposited films, preventing difficulties such as impact marks from being caused in the substrate in the course of its transport by any particles due to dust floating in the bath or film having peeled from the electrode.
To achieve the above objects, as a preferred embodiment, the present invention provides a process for forming a zinc oxide film in an electrodeposition bath provided therein with a continuous-length substrate, an opposing electrode and a back-side film adhesion preventive electrode, to form the zinc oxide film on the continuous-length substrate while transporting the continuous-length substrate under electrification so made as to be in the relation of potential which stands back-side film adhesion preventive electrode less than continuous-length substrate less than opposing electrode; the process comprising the step of removing particles from the surface of the continuous-length substrate. It also provides a process for producing a photovoltaic device, having at least the steps of forming a zinc oxide film by the above zinc oxide film formation process, and forming a semiconductor layer on the zinc oxide film.
As a preferred embodiment, the present invention also provides an apparatus for forming a zinc oxide film, comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a continuous-length substrate, holding the substrate thereon, an opposing electrode, a back-side film adhesion preventive electrode, and a power source and wiring for so making electrification as to be in the relation of potential which stands back-side film adhesion preventive electrode less than continuous-length substrate less than opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the continuous-length substrate. It also provides an apparatus for producing a photovoltaic device having at least the above apparatus for forming a zinc oxide film, and an apparatus for forming a semiconductor layer by plasma CVD to form the semiconductor layer on the zinc oxide film.
As another preferred embodiment of the process, the present invention provides a process for forming a zinc oxide film on a continuous-length substrate while transporting the continuous-length substrate in an electrodeposition bath; the process comprising the step of removing particles from the surface of the continuous-length substrate.
As still another preferred embodiment of the process, the present invention provides a process for forming a zinc oxide film, comprising the steps of transporting a substrate in an electrodeposition bath, and forming the zinc oxide film on the substrate; the process further comprising the step of removing particles from the surface of the substrate.
As another preferred embodiment of the apparatus, the present invention provides an apparatus for forming a zinc oxide film, comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a continuous-length substrate, holding the substrate thereon, and an opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the continuous-length substrate.
As still another preferred embodiment of the apparatus, the present invention provides an apparatus for forming a zinc oxide film, comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a substrate, holding the substrate thereon, and an opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the substrate.
In addition, without limitation to the process and apparatus for forming a zinc oxide film, the present invention also provides an electrodeposition process, and an electrodeposition apparatus, for forming other film.
Stated specifically, the present invention also provides;
an electrodeposition process for forming a film in an electrodeposition bath provided therein with a continuous-length substrate, an opposing electrode and a back-side film adhesion preventive electrode, to form the film on the continuous-length substrate while transporting the continuous-length substrate under electrification so made as to be in the relation of potential which stands back-side film adhesion preventive electrode less than continuous-length substrate less than opposing electrode; the process comprises the step of removing particles from the surface of the continuous-length substrate;
an electrodeposition process for forming a film on a continuous-length substrate while transporting the continuous-length substrate in an electrodeposition bath; the process comprising the step of removing particles from the surface of the continuous-length substrate;
an electrodeposition process comprising the steps of transporting a substrate in an electrodeposition bath, and forming a film on the substrate; the process further comprising the step of removing particles from the surface of the substrate;
an electrodeposition apparatus comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a continuous-length substrate, holding the substrate thereon, an opposing electrode, a back-side film adhesion preventive electrode, and a power source and wiring for so making electrification as to be in the relation of potential which stands back-side film adhesion preventive electrode less than continuous-length substrate less than opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the continuous-length substrate;
an electrodeposition apparatus comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a continuous-length substrate, holding the substrate thereon, and an opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the continuous-length substrate; and
an electrodeposition apparatus comprising an electrodeposition tank for holding therein an electrodeposition bath, a mechanism for transporting a substrate, holding the substrate thereon, and an opposing electrode; the apparatus further comprising a mechanism for removing particles from the surface of the substrate.
More preferred embodiments of the process of the present invention may include;
an embodiment in which the particles are removed by means of a member comprising an insulating material;
an embodiment in which the particles are removed by means of a member so disposed that its longitudinal direction is in a direction not being at right angles to the transport direction of the continuous-length substrate; the member being brought into contact with the continuous-length substrate to make the particles move in a direction different from the transport direction of the continuous-length substrate;
an embodiment in which the particles are particles having come off from the back-side film adhesion preventive electrode;
an embodiment in which the particles are dust floating in the bath;
an embodiment in which the particles are removed by convection of the electrodeposition bath;
an embodiment in which the electrodeposition bath is convected in a direction of convection which is different from the transport direction of the continuous-length substrate, to make the particles move in a direction different from the transport direction of the continuous-length substrate;
an embodiment in which the particles are removed by spraying a liquid or blowing a gas on the continuous-length substrate;
an embodiment in which the liquid or gas is sprayed or blown in a direction different from the transport direction of the continuous-length substrate, to make the particles move in a direction different from the transport direction of the continuous-length substrate;
an embodiment in which the particles are removed after the continuous-length substrate has passed a region where the continuous-length substrate faces the back-side film adhesion preventive electrode and before the continuous-length substrate comes into contact with a mechanism for transporting the continuous-length substrate;
an embodiment in which the mechanism for transporting the continuous-length substrate is a roller.
More preferred embodiments of the apparatus of the present invention may also include;
an embodiment in which the mechanism for removing particles comprises a member comprising an insulating material;
an embodiment in which the mechanism for removing particles comprises a member which is so disposed that its longitudinal direction is in a direction not being at right angles to the transport direction of the continuous-length substrate, and comes into contact with the continuous-length substrate;
an embodiment in which the mechanism for removing particles is a mechanism for causing convection of the electrodeposition bath;
an embodiment in which the mechanism causing convection of the electrodeposition bath causes the electrodeposition bath to convect in a direction different from the transport direction of the continuous-length substrate;
an embodiment in which the mechanism for removing particles comprises a mechanism for spraying a liquid or blowing a gas on the continuous-length substrate;
an embodiment in which the direction in which the liquid or gas is sprayed or blown is different from the transport direction of the continuous-length substrate;
an embodiment in which the mechanism for removing particles is provided between a region where the continuous-length substrate faces the back-side film adhesion preventive electrode and a mechanism for transporting the continuous-length substrate with which mechanism the continuous-length substrate comes into contact first after the continuous-length substrate has passed that region; and
an embodiment in which the mechanism for transporting the continuous-length substrate is a roller.